Fossil Evidence on Origin of the Mammalian Brain

Abstract

Many hypotheses have been postulated regarding the early evolution of the mammalian brain. Here, x-ray tomography of the Early Jurassic mammaliaforms Morganucodon and Hadrocodium sheds light on this history. We found that relative brain size expanded to mammalian levels, with enlarged olfactory bulbs, neocortex, olfactory (pyriform) cortex, and cerebellum, in two evolutionary pulses. The initial pulse was probably driven by increased resolution in olfaction and improvements in tactile sensitivity (from body hair) and neuromuscular coordination. A second pulse of olfactory enhancement then enlarged the brain to mammalian levels. The origin of crown Mammalia saw a third pulse of olfactory enhancement, with ossified ethmoid turbinals supporting an expansive olfactory epithelium in the nasal cavity, allowing full expression of a huge odorant receptor genome.

Endocranial morphology of PalaeocenePlesiadapis tricuspidensand evolution of the early primate brain

“…The olfactory bulbs are large, joined for most of their length and separated from the cerebral hemispheres by a shallow circular fissure (figures 1b and 2a), a primitive condition for mammals [21] also observed in other plesiadapiforms [9,10]. Contrary to other known plesiadapiforms, the olfactory bulbs of P. tricuspidens are only slightly inflated dorsally, and in distinction from most primates, they are overlain by a thick roof of bone (figure 1a,b; as in P. cookei; electronic supplementary material, figure S2).…”

From chemotaxis to the cognitive map: The function of olfaction

“…A recent study that used high-resolution X-ray computed tomography was able to identify three transitions in which early Jurassic mammals showed a significant and sudden increase in absolute brain size (54). At all three transitions, the increase in brain size could be ascribed primarily to increases in absolute OB and olfactory cortex size.…”

“…At all three transitions, the increase in brain size could be ascribed primarily to increases in absolute OB and olfactory cortex size. The authors conclude, "but at its start, the brain in the ancestral mammal differed from even its closest extinct relatives specifically in its degree of high-resolution olfaction, as it exploited a world of information dominated to an unprecedented degree by odors and scents" (54). The alternative OS explanation is that this is evidence of mammals evolving more sophisticated spatial cognitive abilities, with increases in OB size accompanied by increases in hippocampal size and olfactory cortex size with eventual increases in IS.…”

Decreasing sleep requirement with increasing numbers of neurons as a driver for bigger brains and bodies in mammalian evolution

“…Early mammals were very small [31], and thus presumably had a very small number of cortical neurons [30]. The strong trend towards the appearance of species with increasing numbers of neurons in mammalian evolution [30] raises the question of how daily sleep requirement would be impacted by increasing numbers of neurons, and possibly have evolved in parallel.…”

“…As a consequence, the scaling of the ratio of cortical neuronal density to surface area is expected to occur at different rates in primate and non-primate evolution, a hypothesis also examined here. Furthermore, because early mammals were very small [31], with very few neurons, and the number of cortical neurons tended to increase in mammalian evolution [30], I also determine how increasing numbers of neurons correlate with the total number of daily sleep hours, to gain insight into the evolution of sleep. Finally, I examine whether the ontogenetic decrease in sleep hours is also best explained by a decreasing ratio of neuronal density per surface area in the rat, by making use of published data on the ontogeny of sleep in the rat [5] and of our own data on the developmental changes in the cellular composition of the rat brain ( [32]; data available in table 2).…”